This invention relates to radiation-curable organopolysiloxane-containing release coating compositions, the use of such release coating compositions to coat a substrate, and release coated substrates thereof.
Radiation-curable silicone coatings for release of pressure-sensitive adhesives are well known. In the 1980""s, two radiation-curable silicone release systems appeared on the market. One system was a radiation-curable acrylate-silicone copolymer system curable by a free-radical mechanism under both ultraviolet and electron beam radiation. Such systems are disclosed, for example, in U.S. Pat. Nos. 4,201,808; 4,568,566; 4,678,846; 5,494,979; 5,510,190; 5,552,506; 5,804,301; 5,891,530; and 5,977,282.
The other system was an ultraviolet curable system curing by a cationic mechanism and based on epoxy silicone polymers. Epoxy silicone polymer systems are disclosed, for example, in U.S. Pat. Nos. 4,421,904; 4,547,431; 4,952,657; 5,217,805; 5,279,860; 5,310,601; 5,340,898; 5,360,833; 5,650,453; 5,866,261; and 5,973,020.
Each of these release coating systems exhibits a specific range of release properties, some developed for specific needs or applications. However, there are other applications or uses where the specific release properties of these prior art systems are not ideal. There is, therefore, a need to provide a radiation-curable release system that can easily vary the range of properties of the release compositions, such as to raise or lower the release force and to extend the release performance of the system. There is also a need for such radiation-curable release systems that provide better anchorage of the release coating on a film or liner substrate and which provides better fluid coating rheology. A further need is to provide a radiation-curable release composition that exhibits improved polymer compatibility.
One or more objects of this invention is obtained by a radiation-curable release composition comprising:
(a) from about 50 to about 100 parts by weight of an organopolysiloxane of the formula (I) 
xe2x80x83wherein R is CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94, n is 5 to 15, and p is 50 to 150;
(b) from 0 to about 50 parts by weight of one or more organopolysiloxanes comprising organopolysiloxanes selected from the group consisting of those of formulae (II) and (III) 
xe2x80x83wherein R is CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94, m is 1 to 10, and q is 151 to 300; and 
xe2x80x83wherein R is CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94, and r is 20 to 500;
(c) from 0 to about 10 parts by weight of an organopolysiloxane of formula (IV) 
xe2x80x83wherein R is CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94Oxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94, s is 1 to 10, and t is 301 to 1000;
(d) from 0 to about 10 parts by weight of at least one additive for modifying the adhesion and flexibility properties of the composition, the at least one additive selected from the group consisting of acrylate and vinyl ether monomers; and
(e) from 0 to about 10 parts by weight of a photoinitiator.
For easier release, at least one of component (b) or (c) is preferably present in the composition.
A further aspect of the invention comprises a method for producing a release coating on a substrate by applying the aforedescribed radiation-curable release composition to a substrate and curing the coating on the substrate by exposing the coating to radiation. A still further aspect of this invention comprises a release coated article comprising a substrate which has been coated with the radiation-curable release composition and has been cured by exposure to radiation.
The organopolysiloxane polymers of formulae (I) to (IV) of this invention can be prepared according to processes generally known in the art. For example, an organopolysiloxane polymer can be reacted with allyl glycidyl ether in a platinum catalyzed hydrosilation reaction to produce an epoxy functional organopolysiloxane, and the resulting epoxy functional organopolysiloxane is reacted with acrylic acid in the presence of a Cr111 catalyst to produce the organopolysiloxane polymers of formulae (I) to (IV) of this invention.
While the radiation-curable release compositions containing the organopolysiloxane polymers of this invention may be utilized without component (d), in those occurrences where the organopolysiloxane polymers by themselves do not have sufficient inherent properties to strongly anchor or adhere onto a substrate or liner, especially on plastic films, such as polyesters, polyolefins and the like, an adhesion and flexibility modifying component (d) may be employed.
Any suitable acrylate monomer containing one or more functional acrylate groups may be employed as component (d). As examples of such suitable acrylate monomers, there may be mentioned the following: ethylhexyl acrylate, stearyl acrylate, tetrahydrofurfuryl acrylate, lauryl acrylate, isodecyl acrylate, 2(2-ethoxyethoxy)ethyl acrylate, cyclohexyl acrylate, isooctyl acrylate, tridecyl acrylate, isobornyl acrylate, caprolactone acrylate, alkoxylated phenol acrylates, 1,3-butylene glycol diacrylate, 1,4-butanediol diacrylate, diethylene glycol diacrylate, neopentyl glycol diacrylate, tetraethylene glycol diacrylate, triethylene glycol diacrylate, dipropylene glycol diacrylate, alkoxylated hexanediol diacrylates, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, propoxylated glycerol triacrylate, pentarythritol triacrylate, pentaerythritol tetraacrylate, di-trimethylolpropane tetraacrylate, and di-pentaerythritol pentaacrylate.
Preferred as additive (d) is a mixture of acrylic monomers, such as for example, a 60/40 weight blend of dipentaerythritol pentaacrylate and 1,6-hexanediol diacrylate.
Any suitable vinyl ethers can be included as component (d) in the radiation-curable release coating of the present invention, and these include ethers containing one or more vinyl groups. The vinyl ethers can copolymerize with the acrylates and provide low viscosity properties to the mixtures and flexibility to the cured coating compositions. Examples of suitable vinyl ethers include ethyl vinyl ether, butyl vinyl ether, hydroxy butyl vinyl ether, cyclohexyl vinyl ether, 2-ethylhexyl vinyl ether, octyl vinyl ether, decyl vinyl ether, dodecyl vinyl ether, octadecyl vinyl ether, cyclohexane dimethanol monovinyl ether, phenyl vinyl ether, 1,6-hexanediol divinyl ether, 1,4-cyclohexane dimethanol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, tetraethylene glycol divinyl ether, dipropylene glycol divinyl ether, tripropylene glycol divinyl ether, tetrapropylene glycol divinyl ether, and the propenyl ether of propylene carbonate. Ethers with more than one vinyl group are preferred.
The radiation-curable release compositions of the present invention optionally may contain at least one photoinitiator, component (e). Generally no photoinitiator is required when the composition is to be cured under ionizing radiation, such as electron beam irradiation. However, when the curing is to be done by ultraviolet light, the amount of photoinitiator included in the compositions of the invention may range up to about 10%, more often up to about 5% by weight based on the total weight of the radiation-curable composition. A photoinitiator is incorporated into the curable compositions when compositions are to be cured by exposure to non-ionizing radiation such as ultraviolet light.
Suitable photoinitiators include those compounds that produce free radicals which operate to polymerize and cure the compositions used in this invention. Examples of photoinitiators which may be used in combination with ultraviolet light include, for example, benzyl ketals, benzoin ethers, acetophenone derivatives, ketoxime ethers, benzophenone, benzo or thioxanthones, and the like. Specific examples of photoinitiators include: 2,2-diethoxyacetophenone, 2- or 3- or 4-bromoacetophenone, benzoin, benzophenone, 4-chlorobenzophenone, 4-phenylbenzophenone, benzoquinone, 1-chloroanthroquinone, p-diacetyl-benzene, 9,10-dibromoanthracene, 1,3-diphenyl-2-propanone, 1,4-naphthyl-phenyl ketone, 2,3-pentenedione, propiophenone, chlorothioxanthone, xanthone, fluorenone, and mixtures thereof. As examples of commercially available photoinitiators of this type, there can be mentioned the photoinitiators available from the Ciba Geigy Corporation of Hawthorne, N.Y. under the tradenames of Darocur(copyright) 1173 and Irgacure(copyright) 500 and the photoinitiators available from Sartomer Co. under the tradename Esacure KIP100F.
The radiation-curable release compositions of the present invention are produced by mixing the aforementioned components of the composition. The components may be mixed at room temperature with stirring. In some instances, it may be desirable to employ mild heating to facilitate mixing. However, the main silicone components are very compatible when blended into one another and generally do not require them to be maintained under continuous stirring and heat to keep the system homogeneous as is usual with prior art formulations.
The radiation-curable compositions of the present invention can be stabilized against premature polymerization during storage by the addition of conventional polymerization inhibitors, such as hydroquinone, monomethylether of hydroquinone, phenothiazine, di-t-butyl paracresol, and the like. Amounts of 0.1 weight percent or less of the stabilizers are generally effective.
The radiation-curable release compositions of the invention generally are applied to a substrate prior to curing. The compositions may be applied to a substrate as a coating by any conventional means known in the coating art, such as roller coating, curtain coating, brushing, spraying, reverse roll coating, doctor knife, dipping, die coating and the like.
A wide variety of substrates can be coated with the radiation-curable release compositions of the invention. These compositions can be applied to any suitable substrate when it is desirable to modify the release properties of a surface of the substrate. For example, the compositions of the present invention can be employed to form release coatings on substrates, such as paper, vinyl, polyvinyl chloride, and polyester polyolefin films, non-woven fabrics, glass, steel, aluminum, and the like. Included among the types of paper which can be used is clay coated paper, glassinc paper, polymer coated paper, paperboard from straw, bark, wood, cotton, flax, cornstalks, sugarcane, bagasse, bamboo, hemp, and similar cellulose materials prepared by such processes as the soda, sulfite or sulfate (Kraft) processes, the neutral sulfide cooking process, alkali-chlorine processes, nitric acid processes, semi-chemical processes, and the like. Examples of papers which can be utilized as substrates in preparing the composite laminates of the present invention include Kraft papers such as 40-pound and 50-pound bleached Kraft papers, 41-pound offset grade bleached Kraft paper, and the like.
The amount of radiation-curable release compositions of the invention applied to the various substrates will vary depending upon the characteristics of the substrate, the properties desired in the release coating, the radiation source utilized, and the particular formulation of the release composition. Generally, it is desired to apply the least amount of coating to obtain the desired result. Thus, applied coating weights may, depending on the substrate and intended use, range from about 0.1 to about 10 or more grams/m2.
The release compositions of the present invention can be cured by exposure to known forms of radiation, especially ultraviolet light or ionizing radiation, such as electron beam radiation. One of the advantages of using ultraviolet radiation to effect cure of the composition is that polymerization takes place rapidly at ambient temperature, and heating is not necessary.
Curing of the release compositions of the present invention can be effected in a continuous manner by passing the release-coated substrate through radiation equipment which is designed to provide the coated substrate with sufficient residence time to complete the cure of the coating. Curing is generally to be carried out in the presence of the lowest concentration of oxygen possible, typically at an oxygen concentration of less than 100 ppm, preferably less than 50 ppm. Curing is generally conducted in an inert atmosphere, such as nitrogen or argon. The length of exposure necessary to cure the release compositions of the present invention varies with such factors as the particular formulation used, type and wavelength of radiation, dose rate, energy flux, concentration of photoinitiator (when required), the atmosphere and thickness of the coating. A total dosage of from about 0.2 to 10 megarads, preferably below 4 megarads is sufficient to cure the silicone release compositions by electron beam. For UV cure of the silicone release compositions, a UV dose in the range of from about 0.1 to about 0.5 joules is generally sufficient. The exposure time is generally quite brief and curing is completed in about 0.1 to about 3 seconds. The actual exposure time required to give proper curing for various release compositions can be readily determined by one skilled in the art.
The invention is illustrated by, but not limited to, the following examples.